Thoracic aortic aneurysms, a majority of which occur in the ascending aorta, have significant mortality risk and are thus a major health concern. The primary risk in ascending thoracic aortic aneurysm (aTAA) is that of aortic dissection (splitting the aortic wall) with subsequent damage to coronary vessels and/or the aortic valve, or possibly rupture of the aorta itself. Surgical repair has its own risks, and the current state of the art, based on correlation between aTAA diameter and likelihood of rupture, is statistical, meaning that some patients who do not have surgery die from aneurysm complications, and others undergo a dangerous surgery when conservative treatment would suffice. For better risk assessment, we must understand what features of an aneurysm (or a pre-aneurysmal dilatation) are most threatening. We propose to develop a predictive, multiscale model of the remodeling, dissection, and possible rupture of an aTAA. The model will bridge two scales: the (continuous) vessel scale, capturing the gross shape of the aneurysm, and the (discrete) cell/lamellar scale, accounting for elastin and collagen in an elastic lamella of the vessel wall, an idealized smooth muscle cell, and interlamellar connections. The mechanical response of these small-scale elements will be fully coupled to the macroscopic scale. Because the microscale model will treat individual elements separately and structurally, we will be able to impose more complex and realistic remodeling rules than current continuous, constrained-mixture models. For example, we will be able to introduce collagen deposition by the smooth muscle cells based on the stretch of cytoskeletal elements, we will be able to degrade individual collagen fibers rather than introducing treating the problem in terms of a mass density, and we will be able to account for complex deformations that arise from the non-cylindrical geometry of the aTAA. This approach is the natural and necessary next generation following on the last three to four decades of continuum-level remodeling laws. The multiscale model will be parameterized by comparing model results to the experimental data, and once properly specified, the model will be used to generate and test hypotheses about the nature of aTAA growth and rupture, such as exploring the specific role of interlamellar connections or different possible remodeling rules. This project will provide new insight into the mechanisms by which aTAA?s grow and fail, and it will also serve as a potential paradigm for other studies of remodeling in the vascular system and beyond.